1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to, a liquid crystal display panel and a fabrication method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing a gravity defect in the liquid crystal display panel.
2. Discussion of the Related Art
In general, liquid crystal display (LCD) devices use optical anisotropy and polarization properties of liquid crystal molecules to generate a desired image. In particular, liquid crystal molecules can be aligned in a specific orientation, which can be controlled by applying an electric field across the liquid crystal molecules. The liquid crystal display generally includes a liquid crystal display panel having liquid crystal cells arranged in a matrix-like manner and a driving circuit for driving the liquid crystal display panel.
In addition, the liquid crystal display panel also includes pixel electrodes for applying electric field to each of the liquid crystal cells and a reference electrode, i.e., a common electrode. Generally, the pixel electrode is formed on a thin film transistor (TFT) array substrate in the liquid crystal cells, and the common electrode is formed on another substrate, i.e., a color filter array substrate. Each of the pixel electrodes is connected to a TFT, which is used as a switching device. Thus, the pixel electrodes along with the common electrode drive the liquid crystal cells to transmit light in accordance with a data signal supplied through the TFTs.
FIG. 1 is an expanded perspective view of a liquid crystal display panel according to the related art. In FIG. 1, a liquid crystal display panel includes an upper array substrate 10 and a lower array substrate 20 with a liquid crystal material 8 formed therebetween. The upper array substrate 10 is commonly referred to as a color filter array substrate and has a black matrix 2, a color filter 4, common electrode 6 and an upper alignment layer (not shown) formed on a first substrate 1. The color filter 4 includes color filters of red R, green G and blue B to transmit light of specific wavelength ranges, thereby displaying color lights. The black matrix 2 is formed between adjacent color filters 4 to absorb the light incident from the adjacent cells, thereby preventing a color contrast from being deteriorated.
The lower array substrate 20 is commonly referred to as a thin film transistor (TFT) array substrate and has data lines 18, gate lines 12 and a lower alignment layer (not shown) formed on a second substrate 21. The data lines 18 and the gate lines 12 intersect each other, thereby defining a plurality of cell areas. A gate insulating film (not shown) is formed between the data lines 18 and the gate lines 12. A TFT 16 is formed at each intersection between the data lines 18 and the gate lines 12. In particular, the TFT 16 includes a gate electrode connected to a respective one of the gate lines 12, a source electrode connected to a respective one of the data lines 18, and a drain electrode facing the source electrode with a channel region that includes an active layer and an ohmic contact layer. The TFT 16 is electrically connected to the pixel electrode 14, such that the TFT 16 responds to a gate signal from the respective gate line 12 to selectively supply the data signal from the respective data line 18 to the pixel electrode 14.
The pixel electrode 14 is located at each of the cell areas and includes a transparent conductive material having high light transmissivity. The pixel electrode 14 generates a potential difference with the common electrode 6 by the data signal supplied through the drain electrode of the TFT 16. The potential difference causes the liquid crystal material 8 to rotate by dielectric constant anisotropy. Accordingly, the light incident on the liquid crystal panel from a light source (not shown) is transmitted in accordance with the data signal.
FIG. 2 is a planar view of the liquid crystal display panel shown in FIG. 1. As shown in FIG. 2, the liquid crystal display panel includes a sealant 55 formed on one of the upper array substrate 10 and the lower array substrate 20 (shown in FIG. 1) to bond the upper and lower array substrates 10 and 20 to each other. The sealant 55 includes a glass fiber and is spread over a peripheral region of the liquid crystal display panel, such that the liquid crystal material 8 (shown in FIG. 1) is formed inside the sealant 55.
FIG. 3 is a flow chart illustrating a fabricating method of a liquid crystal display panel according to the related art. As shown in FIG. 3, at step S2, an upper array substrate and a lower array substrate are provided. For example, the upper array substrate 10 and the lower array substrate 20 (shown in FIG. 1) may be separately prepared, i.e., forming the black matrix 2, the color filter 4, and the common electrode 6 on the first substrate 1, and forming the data lines 18, the gate lines 12, the TFTs 16 and the pixel electrodes 14 on the second substrate 2.
Then, at step S4, an upper alignment film and a lower alignment film are formed on the upper and lower array substrates, respectively. For example, the upper alignment film is formed on the first substrate 1 over the black matrix 2, the color filter 4, and the common electrode 6 (shown in FIG. 1), and the lower alignment film is formed on the second substrate 21 over the data lines 18, the gate lines 12, the TFTs 16 and the pixel electrodes 14 (shown in FIG. 1).
Further, at step S6, a sealant is applied on one of the upper array substrate and the lower array substrate. For example, the sealant 55 (shown in FIG. 2) is formed on one of the first substrate 1 and the second substrate 21 (shown in FIG. 1). Then, at step S8, the upper and lower array substrates are bonded to each other. For example, the first and second substrates 1 and 21 (shown in FIG. 1) are bonded to each other by the sealant 55 (shown in FIG. 2).
In addition, at step S10, a liquid crystal material is injected into a space inside the sealant between the upper and lower array substrates. For example, the liquid crystal material 8 is injected between the first and second substrates 1 and 21 (shown in FIG. 1).
FIG. 4 is a diagram showing a gravity defect in the liquid crystal display panel shown in FIG. 1. As shown in FIG. 4, the liquid crystal display panel includes a spacer 22 to maintain a cell gap between the first and second substrates 1 and 21. In particular, the liquid crystal material 8 is injected in the cell gap maintained by the spacer 22 inside the sealant 55 between the first and second substrates 1 and 21. However, when in a high temperature environment, the liquid crystal material 8 may expand. In particular, as voltages are applied to the liquid crystal display panel, the temperature inside the liquid crystal display panel increases. Further, the temperature inside the liquid crystal display panel is increased by the heat generated from a back light device.
As the liquid crystal material 8 expands, the liquid crystal material 8 pushes the first and second substrates 1 and 21 outwardly and the spacer 22 is separated from the first substrate 1 or the second substrate 21. As a result, the cell gap between the first and second substrates 1 and 21 becomes larger than the spacer 22 and the liquid crystal material flows downwardly because of gravity, thereby generating spots at the lower corner area of the liquid crystal display panel by the difference of the gray, which is called a gravity mura, and resulting in a defect. Accordingly, the cell gap of the liquid crystal display panel according to the related art becomes uneven and provides a poor image.